Many modular switches include a plurality of cards which are connected by a backbone bus with a limited bandwidth. Generally, a box of the modular switch comprises a plurality of slots for receiving cards, the slots being connected by the backbone bus.
When the number of slots in the modular switch is relatively large and/or the amount of bandwidth needed by each card is large, the bandwidth of a bus which allows all the cards to transmit their entire capacity concurrently when all the slots are full with cards, may be very large. A bus with such a large bandwidth is very expensive and is not generally necessary as substantially nobody uses all the cards at maximal capacity at the same instant. Therefore, it is common to use a bus with a lower bandwidth. It is required, however, that the use of a lower bandwidth bus have minimal impact on the user of the switch.
Some modular switches divide the bus into fixed portions which are assigned to the various slots of the switch. In a first variation, all the bus portions have the same bandwidth which is lower than the maximal bandwidth capacity of the cards. The user therefore must make sure that no card reaches its capacity, or the traffic of the card will be blocked. In a second variation, some slots are assigned a large bus portion which is sufficient for the card it carries even in its full capacity, and other slots are assigned smaller bus portions. The bus portions assigned to the slots are either predefined at the manufacturing of the switch or are configured by a system manager in charge of the switch. These switches are limiting as they require that the cards be placed in specific slots or that the switch be configured each time a card is moved.
In some switches, the bus is divided into sub-buses of the size of the maximal capacity of the cards. The sub-buses are allocated to the cards using time domain multiplexing. Such multiplexing, however, is wasteful as it gives all the cards the some amount of bandwidth regardless of their needs. In addition, this solution adds delay to packets received by a card when it is not its turn to use the bus. In some buses which use time domain multiplexing, the bus is divided into slots, and each time a transmitter needs to transmit data it requests an amount of slots. This solution, however, is too slow and complex for fast switches.
In many organizations it is desired to limit the flow of data between computers. For example, it may be desired that engineers of a first project not have access to files describing a second project. In such cases separate networks are laid out for the different groups which need to talk to each other. In addition, for redundancy it is sometimes desired to have two separate networks with limited or no interconnections. Thus, if a device connected to one of the networks blocks the usage of the network to which it belongs, the other network may still be used. Furthermore, in some cases it is desired to form separate connections using different protocols, such as Ethernet and ATM, which cannot be interconnected without signal conversions. A simple solution is to create separate networks using a plurality of different switches which are not interconnected. This solution is, however, expensive and inflexible.
Another solution known in the art is to define separate virtual local area networks (VLANs) which are implemented on a single network. Each packet transmitted through a VLAN supporting network carries TAG information which states the identities of the VLANs to which the packet belongs. The packet is passed only through ports which are configured to receive packets of the specific VLAN. The problem with this solution is that the packets themselves carry the direction information, and if this information is changed, intentionally or unintentionally, the packets may be passed through the wrong port. Also, changing the configuration of the VLANs of the ports is generally allowed separately by each computer, requiring separate protections on each computer to prevent unauthorized changing of the definitions of the VLANs of the ports. An emerging standard for VLANs is described in “Draft Standard P802.1Q/D9, IEEE Standards for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks”, 1998, the disclosure of which is incorporated herein by reference.
There exist boxes which include within them both an Ethernet switch and an ATM switch. Some of these boxes have a predetermined first group of slots for ATM cards and a predetermined second group for Ethernet cards. These boxes, however, define a fixed border between the ATM and Ethernet switches and do not allow shifting of cards from one switch to the other, according to the current needs of the user. Others of these boxes, such as of the Meritage switch available from Lucent, include two busses, one for ATM cards and one for Ethernet cards. Such a switch requires a bus of double size than a switch which supports only a single protocol.
There also exist cards which perform conversion of signals from a first protocol to a different protocol of the bus. For example, an ATM card which receives ATM signals may convert its signals to Ethernet signals for transmission over an Ethernet bus. Such conversion cards are naturally more complex than simple single protocol cards.
Some modular switches in accordance with the token ring protocol include a plurality of slots into which cards are inserted. By leaving empty slots between groups of cards, a plurality of separate rings are formed.